CHEMLAB Manual

Experiment 9: Determination of %Ascorbic Acid

• Ascorbic acid (vitamin C) is analyzed using iodine as an oxidizing titrant.
• Iodine oxidizes the enediol functional group of ascorbic acid to an alpha diketone.
• The reaction between ascorbic acid and iodine is an oxidation-reduction (redox) reaction.

Experiment 8: Standardization of I2

• Redox titration is a type of titration where the reaction between the analyte and titrant is an oxidation-reduction reaction.
• Iodine is a weak oxidizing agent and is useful for analyzing strong reducing agents.
• There are two types of titrations that use iodine: iodimetry and iodometry.
• Iodimetry is a direct method that uses a standard I2 solution in titrating an easily oxidized substance.
• Iodometry is an indirect method used for analyzing oxidizing agents.
• The endpoint of the titration is indicated by the appearance of a blue color using a starch indicator.

Experiment 7: Determination of Purity of an Impure Soda Ash

• Soda ash is a common name for the inorganic compound, sodium carbonate (Na2CO3).
• Soda ash is widely used as a cleaning agent, a component of dry soap powders, and in baking powder.
• Soda ash is water-soluble and forms slightly alkaline solutions when dissolved in water.
• The purity of soda ash is determined by titrating with a standard acid, usually HCl.
• The endpoint is signaled when the indicator (methyl orange) turns light orange.

Experiment 6: Determination of the Acid Content of Commercial Vinegars

• The total acid content of a vinegar or a wine is determined by titration with a standard base.
• The acid content of vinegar is usually reported in terms of acetic acid, even though other acids are present.
• The molar mass of acetic acid is 60.05 g/mol.

Experiment 5: HCl vs NaOH

• A 0.25 M hydrochloric acid (HCl) solution is prepared by diluting the concentrated reagent.
• The diluted acid solution is then standardized against a primary or secondary standard base.
• The exact concentration of HCl solution is calculated by measuring the volume of the HCl solution dispensed from the burette that is necessary to react completely with an accurately measured volume of previously standardized NaOH solution.
• The endpoint is detected visually using a phenolphthalein indicator.

Experiment 4: NaOH vs KHP

• Standardization is the process of determining a solution's exact concentration (molarity or normality).
• There are two types of analytical standards: primary standards and secondary standards.
• Primary standards are reagents that contain known amounts of analyte and have a known stoichiometry, purity, and stability.
• Secondary standards are reagents that do not meet the criteria for a primary standard.
• The concentration of a secondary standard is determined relative to a primary standard.
• A 0.25 M sodium hydroxide (NaOH) solution is prepared and standardized by titrating it against an accurately weighed primary standard acidic substance, potassium hydrogen phthalate (KHP).
• The exact concentration of NaOH solution is calculated by measuring the volume of the NaOH solution dispensed from the burette that is necessary to react completely with a weighed sample of KHP.
• The endpoint is detected visually using a phenolphthalein indicator.

Experiment 9: Determination of %ascorbic acid in an impure substance

• Iodine is used as an oxidizing titrant to analyze ascorbic acid (vitamin C) by oxidizing the enediol functional group to an alpha diketone.
• The reaction leads to the immediate reduction of iodine to iodide if ascorbic acid is present.
• The excess iodine reacts with the starch indicator to form a blue starch/iodine complex, indicating the endpoint of the titration.

Redox Titration

• Redox titration is a type of titration where the reaction between the analyte and titrant is an oxidation-reduction reaction.
• The titrant can be either an oxidizing agent or a reducing agent, and the analyte can be either a reducing agent or an oxidizing agent.
• Iodine is a weak oxidizing agent and is useful for analyzing strong reducing agents.
• Its selectivity makes it a good titrant for analyzing a strong reducing agent in the presence of weaker reducing agents.

Standardization of I2

• Iodine is standardized using iodometric titration with a previously standardized sodium thiosulfate (Na2S2O3) solution.
• Iodine is not very soluble in water, so it is dissolved in moderately concentrated potassium iodide solutions to produce a reasonable concentration.
• Anhydrous sodium thiosulfate is used as an alternative standard to prepare iodine solutions.

Experiment 8: Standardization of I2

• The prepared iodine solution is titrated with a standard sodium thiosulfate solution to a pale straw color, and then the starch indicator is added.
• The mixture turns blue due to the formation of a starch/iodine complex, and then titrated further with sodium thiosulfate solution to the disappearance of the blue color.

Experiment 7: Determination of Purity of an Impure Soda Ash

• Soda ash is a common name for sodium carbonate (Na2CO3), with a molar mass of 106 g/mol.
• It is widely used as a cleaning agent, a component of dry soap powders, and a component of baking powder.
• Soda ash is water-soluble and forms slightly alkaline solutions when dissolved in water.
• The total alkalinity of soda ash is determined by titrating with a standard acid, usually HCl, using methyl orange indicator.

Experiment 6: Determination of the Acid Content of Commercial Vinegars

• The total acid content of a vinegar or wine is determined by titration with a standard base.
• The acid content is reported in terms of acetic acid, the principal acidic constituent, even though other acids are present.
• The molar mass of acetic acid is 60.05 g/mol.

Experiment 5: HCL vs NaOH

• A 0.25 M hydrochloric acid (HCl) solution is prepared by diluting concentrated HCl.
• The diluted acid solution is then standardized against a primary or secondary standard base.
• The exact concentration of HCl solution is calculated by measuring the volume of the HCl solution dispensed from the burette necessary to react completely with a previously standardized NaOH solution.
• The completeness of the titration is detected visually using a phenolphthalein indicator.

Experiment 4: NaOH vs KHP

• Standardization is the process of determining a solution's exact concentration (molarity or normality).
• Primary standards are reagents that contain known amounts of analyte, while secondary standards are reagents that do not meet the criteria for a primary standard.
• A base volumetric solution, specifically a 0.25 M sodium hydroxide (NaOH) solution, is prepared and standardized by titrating it against an accurately weighed primary standard acidic substance, potassium hydrogen phthalate (KHP).
• The exact concentration of NaOH solution is calculated by measuring the volume of the ~0.25 M NaOH solution dispensed from the burette necessary to react completely with a weighed sample of KHP.